You really think it would have rendered the first 3 movies pointless? I don't see that. Terminator needed a kick in the pants, and that would have definitely done it. I guarantee people would have been talking about it more, which means people would have wanted a sequel more. Now, I think we will be lucky if that happens.
I'm a giant fan of the first two movies. And at first that would have felt like sacriledge, but you have to move beyond people's expectations and find something that serves the story better. Look at the Star Wars pre-quels, they were pretty to look at, but felt kind of hollow because you knew what was coming. Terminator involves time travel, so like Star Trek, it had an opportunity to take it in a ballsy new direction and chickened out. You need to pay attention to fans, yes, but sometimes obeying their whims will keep their own beloved property from growing. And Terminator had a chance to grow and expand on the myth, but didn't.